Polarization modulation of an optical signal is a well known technique of transmitting information in optical communication systems, with either direct or coherent detection. Coherent detection systems are of particular interest since, thanks to conversion of the optical carrier into radiofrequency, selectivity of electronic filters can be used in optical transmissions, thus, permitting a more complete exploitation of the band available on the transmission medium in the case of frequency division multiplex communications. Moreover, in the case of optical fiber transmissions at relatively high wavelengths, such as those lying in the second and third transmission windows (1.3-1.6 .mu.m), coherent systems eliminate detector noise problems. The use of polarization modulation in such systems affords the advantage of a sensitivity comparable with that of frequency modulation systems with a band occupancy equal to that of amplitude modulation systems.
Typical examples of such systems are described in U.S. Pat. No. 3,435,229 or in the article "Heterodyne transmission of a 560 Mbit/s optical signal by means of polarization shift keying" by E. Dietrich et al., Electronics Letters, Vol. 23, No. 8, Apr. 9, 1987. In these known systems, the signal emitted by the source is supplied to a modulator which, depending on the logic value of the information signal, modulates the state of polarization of the radiation emitted by the source, for instance by allotting orthogonal polarization states to the two logic levels of the information signal. At the receiving side, the line signal is combined with the signal emitted by a local oscillator, polarised so as to present components having the same polarization as the line signal; the signal resulting from the combination is polarization analyzed, the different components are separately converted into electrical signals and the signals received are suitably demodulated.
In such systems the polarization modulation of the optical carrier requires the use of modulators external to the source. At the transmission rates typical of coherent optical communications systems (from some hundred Mbit/s to some Gbit/s), the modulators commonly used are integrated-optics guide elements, which are generally expensive and moreover give rise to high losses, due to both the attenuation of the guide itself and the coupling between the guide and the possible fiber pig-tail allowing connection to an optical fiber.